Space weather

Lore & Background

Space weather encompasses the dynamic conditions within the Solar System and its heliosphere, driven primarily by the solar wind and the interplanetary magnetic field it carries. Its effects are most notably observed on Earth’s magnetosphere, ionosphere, thermosphere, and exosphere, though it is physically distinct from the terrestrial weather of the lower atmosphere. A defining characteristic is its ability to produce geomagnetic storms and substorms, which can disrupt global communications, damage satellites, cause power surges in long-distance transmission lines, and expose aircraft passengers and crew to increased radiation, particularly on polar routes. The most famous historical example is the solar storm of 1859, which triggered brilliant auroral displays and disrupted telegraph operations worldwide. Aurorae themselves are a visible manifestation of space weather, occurring in a permanent oval-shaped region of luminescence located 15 to 25 degrees in latitude from the magnetic poles. Other key phenomena include solar radio bursts—broad-frequency radio waves emitted during solar flares—and the Van Allen belts, regions of trapped radiation particles discovered in the late 1950s. The solar wind, first directly measured in 1959, and the electric fields it impresses on the high-latitude ionosphere are fundamental drivers of these space weather effects.

Reader's Guide

Space weather is a branch of space physics and aeronomy, or heliophysics, that studies the changing conditions within the Solar System and its heliosphere, particularly the effects of the solar wind on Earth’s magnetosphere, ionosphere, thermosphere, and exosphere. Though physically distinct, it is analogous to terrestrial weather. The term was first used in the 1950s and popularized in the 1990s, later prompting research into “space climate,” the large-scale, long-term patterns of space weather. Historically, auroral displays were observed for centuries without understanding. In 1724, George Graham noted daily deflections of a magnetic compass needle, later attributed to overhead electric currents in the ionosphere and magnetosphere by Balfour Stewart in 1882 and confirmed by Arthur Schuster in 1889. In 1852, Edward Sabine correlated geomagnetic storm frequency with sunspot numbers, revealing a solar-terrestrial link. The 1859 solar storm caused brilliant aurorae and disrupted global telegraphs; Richard Carrington connected it to a solar flare observed near a sunspot group. Kristian Birkeland explained auroral physics through laboratory experiments and predicted the solar wind. Radio’s introduction showed solar weather could cause static; radar jamming during a 1942 solar event led to the discovery of solar radio bursts. The International Geophysical Year advanced research, showing aurorae occur in a permanent oval. Explorer I discovered the Van Allen belts in 1958, and Luna 1 first directly observed the solar wind in 1959. INJUN-5 measured electric fields in the high-latitude ionosphere in 1969, and Triad data later revealed permanent electric currents between the auroral oval and magnetosphere. Space weather phenomena can interfere with or damage communications and weather satellites, disrupt GPS signals, cause surges in long-distance power lines, and expose aircraft passengers and crew to radiation, especially on polar routes. It is recognized as a natural hazard in risk assessments in Ireland, the United Kingdom, and New Zealand.

Did You Know?

• The term 'space weather' was first used in the 1950s and popularized in the 1990s.

The Physical Fabric of the Void

Outer space is, as far as we can measure, the closest thing nature offers to a perfect vacuum. Rather than being truly empty, it is threaded with an ultra-thin plasma of hydrogen and helium, scattered electromagnetic radiation, cosmic rays, neutrinos, magnetic fields, and fine dust. The baseline temperature, set by the residual glow of the Big Bang, sits at just 2.7 kelvins—roughly minus 270 degrees Celsius. Between galaxies, that plasma is so sparse that fewer than one hydrogen atom occupies a cubic metre, yet its kinetic temperature reaches millions of kelvins. This intergalactic medium is believed to hold about half of all ordinary baryonic matter in the observable universe. Local clumps of matter have condensed into stars and galaxies, but even those structures are overwhelmingly empty. The remaining mass-energy budget is dominated by two mysterious components—dark matter and dark energy—that resist direct detection and reshape our understanding of what fills the void.

From the First Expansion to the Observable Cosmos

According to the Big Bang model, the universe began roughly 13.8 billion years ago as an extraordinarily hot and dense state that expanded at a tremendous rate. For the first 380,000 years, matter and radiation were locked together in a searing fog. When the temperature finally dropped enough for protons and electrons to bind into hydrogen—the recombination epoch—photons were released and have been streaming freely ever since. That ancient light, now redshifted into microwaves, is what satellites like the Wilkinson Microwave Anisotropy Probe have mapped to determine the universe's geometry. The result: the observable universe is spatially flat, meaning parallel light rays stay parallel across cosmic distances unless bent by local gravity. Combined with measured mass density and the observed accelerating expansion, this flatness points to a non-zero vacuum energy we call dark energy. On average, the universe contains the equivalent of about 5.9 protons per cubic metre, yet ordinary atoms make up only 4.6 percent of that total, and density varies enormously from the packed cores of galaxies to the near-emptiness of vast intergalactic voids.

Drawing the Line Between Sky and Space

Unlike a mountain range or an ocean trench, outer space has no sharp physical boundary. The convention used in international treaties and aerospace record-keeping is the Kármán line, set at 100 kilometres above sea level, named after Theodore von Kármán, who described the altitude at which atmospheric drag becomes negligible and spacecraft can operate free of it. Below that threshold, portions of the upper stratosphere and mesosphere are sometimes called near space. The legal architecture governing the region above was codified in the Outer Space Treaty, which took effect on 10 October 1967. That treaty bars any nation from claiming sovereignty over celestial territory and instead grants every state the right to explore freely. The terminology itself has a long literary pedigree: John Milton used the bare word space in Paradise Lost in 1667, Alexander von Humboldt applied outer space to astronomy in 1845, and H. G. Wells popularised the phrase after 1901. Yet despite UN resolutions calling for peaceful use, anti-satellite weapons have still been tested in Earth orbit, reminding us that the legal line is not always the practical one.

The Human Reckoning with the Void

The idea that the gap between Earth and the Moon is a vacuum emerged in the 17th century once scientists demonstrated that air pressure falls with altitude. It was not until the 20th century that the true scale of that void became measurable, when the distance to the Andromeda Galaxy was first determined. Physical exploration followed in stages: high-altitude balloon flights opened the door, then crewed rocket flights, and finally Yuri Gagarin's orbit of Earth in 1961 for the Soviet Union. The economic price of launching mass into space remains so steep that human missions have been confined largely to low Earth orbit and the Moon, while uncrewed probes have visited every known planet in the Solar System. The environment itself is hostile: the vacuum and pervasive radiation pose constant threats, and microgravity steadily erodes the human body, causing muscle atrophy and bone loss. Space is, in every measurable sense, a place that resists human presence, and our foothold there remains fragile and expensive.